Improving understanding of seabird bycatch in Scottish longline fisheries and exploring potential solutions

Section 1: A literature review of northern fulmar bycatch rates, bycatch risk factors and potential mitigation approaches relevant to UK demersal longline fisheries.

1.1 Introduction

The northern fulmar is an abundant and widely distributed seabird of the taxonomic family Procellariidae. There are three recognised subspecies: Fulmarus glacialis glacialis breeds in the high Arctic regions of the North Atlantic, Fulmarus glacialis audubonii breeds in the boreal North Atlantic and Fulmarus glacialis rodgersii breeds in the Pacific Arctic (Edwards 2015). Northern fulmars mature between 6 to 10 years old and regularly attain 30 years of age, with some ringed specimens estimated at over 40 years old (Fransson 2017). The species is thought to be monogamous and returns to the same nesting site annually (Hatch and Nettleship 1998).

In the North Atlantic, the breeding season occurs between May and September and during that period adult birds tend to forage relatively close to the nesting site, with a mean foraging distance of 60km reported in Norway by Weimerskirch et al. (2001) but much longer foraging trips can also occur (Edwards et al. 2013). Thaxter et al. (2012) also provided estimates of fulmar foraging ranges based on data from Greenland, Norway and the United Kingdom with a mean of 47.5km and a mean maximum of 400km across the studies considered. Northern fulmar breeding distribution has changed dramatically in the North Atlantic over the last few centuries. Prior to the mid-18^th century, breeding occurred in just a few locations in Iceland and St Kilda in the Western Isles of Scotland (Lloyd et al. 2010). The breeding range then began to expand around the coast of Iceland and to the Faeroe Islands, and a second colony also formed in Scotland on the Shetland Isles. More recently breeding colonies have become established in many countries of Northwest Europe and the species has also spread west across the Atlantic and now breeds along the east coast of Canada.

Outside the breeding season the northern fulmar is a truly oceanic seabird and spends all its time at sea, but the species does not appear to have a well-defined migratory route like some other seabird species (Mallory 2008; Edwards et al. 2013). This wide-ranging behaviour is highlighted by the movements of a GPS tagged individual that spent part of its time in waters around Scotland, the Faeroes and the Norwegian Sea, and part of its time further west to the mid-Atlantic ridge, Newfoundland and the Labrador Strait (Edwards et al. 2013). There were relatively few GPS positions recorded between those two general areas which indicated relatively quick transits between the Mid-Atlantic Ridge (and further west) and Northwest European waters.

Fulmars have a well-developed olfactory system (Wenzel 1986; Fangel et al. 2015) which they use to locate food sources (Nevitt 2008), such as aggregations of copepods in the open ocean (Edwards et al. 2013). They may also use this ability to locate fishing vessels at sea and often forage on discards and offal (Garthe & Hüppop 1994).

Through the 20^th century northern fulmar abundance increased quite rapidly and this has been suggested to be related to whaling activity and increasing trawl effort (Fisher 1966), which led to enhanced foraging opportunities. Studies of fulmar diet suggest that some populations feed predominantly on zooplankton, sandeels or other fishery-independent sources of food but at some colonies a high proportion of the diet may be discards or offal (Hudson & Furness 1988; Ojowski et al. 2001).

The global (all subspecies) northern fulmar population was estimated at about seven million breeding pairs, or twenty million individuals including immature non-breeders (Birdlife International 2021). The European breeding population (which does not include Northwest Atlantic birds) was estimated at about seven million individuals (Birdlife 2015). The UK population was estimated to be about 500,000 breeding pairs during the period 1998 to 2002 (Seabird 2000), which represents a 3% decline in abundance from the previous census that was carried out from 1985 to 1988. However, there is evidence of a steeper decline since 2000. The latest available estimates for 2019, which are based on a sample of colonies rather than a census as was used previously, indicated reductions in the region of 37% since the index began in 1986 (JNCC 2020).

The IUCN Red List (IUCN 2021) currently classifies northern fulmar as “Least Concern” and the overall population trend is considered to be increasing. Similarly, Partners in Flight, a network of over 150 organisations involved in science and policy development, rates the species a 9 out of 20 on the North American Continental Concern Score, indicating a species of low conservation concern. Birds of Conservation Concern 4 which relates to the population status of birds in the UK, Channel Islands and Isle of Man (BoCC4 2015) currently classifies northern fulmar as “Amber”, so the species is of some, but not critical, conservation concern. However, northern fulmar has recently been reclassified as “Endangered” on the European Red List (Birdlife International 2015) as the population trend across much of the region is decreasing.

Reduced catch rates and discarding by EU fishing fleets since the late 1990s may have had a significant impact on the foraging success of several scavenging seabird populations in European waters (Bicknell et al. 2013) and declines in the abundance of the North Atlantic fulmar population since the 1980’s are considered to be at least partially related to reduced foraging opportunities around fishing boats and may represent a re-adjustment to more natural population levels following a period of elevated abundance related to fisheries byproducts (JNCC 2020). It seems likley that trends in fulmar abundance in the North Atlantic over the last century are intricately linked to levels of fishing activity, evolving fishing practises and fisheries management measures.

Preliminary mortality estimates (Northridge et al. 2020) based on limited observer data indicated that northern fulmar bycatch in some UK fishery sectors (nets, midwater trawls and longlines) is likely to be about 5,000 individuals per annum. It is not clear which breeding populations bycaught birds originate from. Most of the estimated UK mortality appears to occur in the offshore longline fishery that targets European hake in waters to the north and northwest of Scotland, with lower-level bycatch estimated to occur in the same fishery when it operates to the west and south of Ireland. Some bycatch mortality also occurs in static net fisheries (Northridge et al. 2020).

This review was compiled for two main purposes. Firstly, to draw together available information on fulmar bycatch rates and risk factors from other demersal longline fisheries in the northern hemisphere to help contextualise and inform our understanding of fulmar bycatch in UK fisheries, and secondly, to provide a summary of potential bycatch mitigation measures that may be of use to industry and Government to help reduce bycatch in the longline fishery in practical, economic and implementable ways.

1.2 Methods

We used academic bibliographic databases (e.g., Biosis, Web of Science); Google Scholar and contract or working group/workshop reports to identify published studies that estimated fulmar bycatch rates or described seabird bycatch mitigation measures that have been tested in demersal longline fisheries. In addition to specific research articles that typically cover a single fishery or mitigation approach, several global reviews of mitigation measures for longline fisheries are also available (Bull 2007; Løkkeborg 2011; Parker 2017). We also directly contacted scientists working on fulmar bycatch in other countries to obtain information from unpublished or ongoing work.

From published work in peer-reviewed journals, Biosis and Web of Science yielded a only a few tens of relevant papers using the search terms “fulmar”, “longline” and “bycatch”. Google Scholar yielded 910 results using the same search terms. We briefly reviewed all of these by scanning titles and abstracts, and initially identified about 200 papers that seemed at least partially relevant. More detailed review of these 200 papers and various reports resulted in a final set of about 90 publications of specific relevance to fulmar bycatch rates and potentially useful mitigation approaches for large scale demersal longlining operations.

1.3 Overview of fulmar bycatch & risk factors from other northern hemisphere longline fisheries

Largely because of their habit of following fishing vessels that are provisioning them with offal, fulmars are vulnerable to becoming caught in some kinds of fishing gear, notably on hooks in longline fisheries where they may attempt to take the bait from hooks as the lines are being set or hauled. Fulmar bycatch on longlines has been reported in many fisheries around the world. Northern fulmars are known to be caught on longlines in Alaska, Canada, Norway, Iceland, and the Faroes as well as in other European longline fisheries. There are relatively few recent estimates of bycatch in most of these fisheries.

Bycatch rates are usually measured in terms of the number of birds per 1000 hooks (Kriegr and Eich 2021). Melvin et al. (2019) showed that fulmar bycatch in Alaskan longline fisheries has declined substantially since the introduction of voluntary mitigation measures in 2002. Prior to 2002, fulmar bycatch rates averaged 0.051 birds per thousand hooks. After the adoption of streamer lines in 2002, the rate fell to 0.01 birds per thousand hooks, a five-fold reduction. The fishery targeted 4 species: sablefish (Anoplopoma fimbria), Pacific halibut (Hippoglossus stenolepis), Pacific cod (Gadus macrocephalus) and Greenland turbot (Reinhardtius hippoglossoides). Halibut was and remains the largest of these Alaskan fisheries.

Recent bycatch mortality estimates for northern fulmar in the Alaskan longline fishery (Krieger and Eich 2021) have fallen to an average of 3067 individuals per year between 2011 and 2020, compared with an average of over 10,000 per year between 1993 and 1999 (Melvin et al. 2001). This decline in mortality is partly due to the introduction of voluntary mitigation measures but also reflects the fact that many vessels switched from using longlines to pots in the sablefish fishery. Annual bycatch of northern fulmar in all Alaskan fisheries (of which 86% are in longline fisheries) was recently estimated at 0.25% of the population of 1.4 million birds and is considered low (Krieger and Eich 2021).

In the Western North Atlantic, Ellis et al. (2013) used observer data from 2002-2006 to estimate annual bycatch of seabirds in the pelagic and demersal longline fisheries in Atlantic Canada. Approximately 230 fulmars were estimated killed annually in these fisheries. Observer data have been collected in the region since 2001, but it seems they have not been fully analysed to date.

In the Eastern North Atlantic, Dunn & Steel (2001) suggested that the Norwegian offshore auto-lining fleet might take around 10,000 seabirds (mostly fulmars) per year, based on mean observed bycatch rates in their study, with no spatial stratification. They point out that there were over 9000 mostly small vessels engaged in some form of longlining in Norway, and so suggested that bird bycatches could have been around 20,000 birds per year in Norway in total, but with a rider that the total “may easily run to 50,000 to 100,000” per year when using more extreme Bycatch Per Unit Effort (BCPUE) figures. They also suggested, based simply on the numbers of vessels operating in Iceland and the Faroes, that the figure of 50,000 to 100,000 would be a conservative estimate for the entire ‘Nordic’ fleet. These speculative estimates have entered the literature more widely.

Anderson et al. (2011) using upper limits for bycatch estimates cited as being from Dunn and Steel (2001) suggested that as many as 140,000 birds per year could be taken in the combined longline fleets of Norway, Iceland and the Faroes. The authors point out that the estimates are from old studies of bycatch per unit effort in the 1980s and 1990s, against which they used more recent estimates of effort.

Fangel et al. (2015) suggested that the Norwegian bycatch of fulmars in the two coastal longline fisheries was around 3300 in the cod and haddock fishery and around 1500 in the Greenland halibut fishery in 2009, based on interview surveys. However, a later study of the Greenland halibut longline fishery alone involving a larger sample of 426 trips involving both interview data and observer data yielded an estimate of around 310 fulmars in total over the years 2012–2014 (Fangel et al. 2017), or about 100 per year. The reasons for the large differences in these estimates are unclear, but very different BCPUE figures were reported in the two studies, and the discrepancy underlines the difficulties in extrapolating from relatively small samples and when bycatches can be highly clumped. As the author’s state: “few yet sometimes extreme bycatch events are … highly influential in controlling the bycatch estimates”.

A small study involving observations of 103 fishing operations from 14 trips in the UK offshore hake longline fishery yielded provisional mortality estimates for fulmar of about 4,500 (95% CI 2200-9100) individuals per year (Northridge et al. 2020), but the authors stressed that the data were too limited (and likely had significant biases) to put much trust in. Estimates of fulmar bycatch in other European longline fisheries are generally lacking.

A summary of available northern fulmar bycatch estimates in longline fisheries is provided in Table 1.

Table 1: Summary of northern fulmar bycatch estimates from other longline fisheries.

Authors	Reference years	Nation	Target species	Region	Approximate annual totals	Type of estimate
Melvin et al. 2001	1993-1999	Alaska USA	Halibut, sablefish, cod	Bering Sea & Aleutian Islands	9000	Observed BCPUE
Melvin et al. 2001	1993-1999	Alaska USA	Halibut, sablefish, cod	Gulf of Alaska	1000	Observed BCPUE
Krieger and Eich 2021	2011-2020	Alaska USA	Halibut, sablefish, cod	All Alaska	3000	Observed BCPUE
Ellis et al. 2007	2002-2006	Canada	Pelagic & demersal	Maritime Provinces	230	Observed BCPUE
Dunn and Steel 2001	1997-1998	Norway	Demersal species	Offshore	10,000	Observed BCPUE
Dunn and Steel 2001	1997-1998	Norway	Demersal species	Inshore	10,000	Speculative
Dunn and Steel 2001	1997-1998	Nordic fleets	Demersal species	Combined	50,000-100,000	Speculative
Anderson et al. 2011	2007	Norway	Demersal species	Combined	6500 (up to 110,000)	Observed BCPUE (& updated effort)
Anderson et al. 2011	2007	Iceland	Demersal species	Combined	7000 (up to 20,000)	Speculative
Anderson et al. 2011	1997-1998	Faroes	Demersal species	Combined	3000 (up to 10,000)	Speculative
Fangel et al. 2015	2009	Norway	Cod/Haddock	Inshore	3300	Interviews - reported trip totals
Fangel et al. 2015	2009	Norway	Greenland Halibut	Inshore	1500	Interviews - reported trip totals
Fangel et al. 2017	2012-2014	Norway	Greenland Halibut	Inshore	100	Interviews and observed BCPUE
Northridge et al. 2020	2010-2018	UK	Hake	Offshore	2200-9100	Observed BCPUE

Bycatch rates, expressed as numbers of birds per thousand hooks are highly variable even within the studies listed above. Dunn and Steel (2001) tabulated recorded fulmar bycatch rates in Norwegian longline fisheries, which varied by two orders or magnitude from 0.01 to 1.75 in different studies. In Canada reported bycatch rates of fulmars appear to be at the lower end of published ranges: although data in Hedd et al. (2016) are not broken down explicitly by seabird species, in fisheries where fulmars dominate the bycatch, recorded seabird bycatch rates range from 0.008 (Atlantic cod) to 0.076 (Greenland halibut) birds per thousand hooks. Fangel et al. 2017 reported bycatch rates in the Greenland Halibut fishery in Norway of 0.294 birds per thousand hooks in 2009 and 0.045 in 2012-14 in the same fishery. Melvin et al. (2019) provide figures of 0.0513 birds per thousand hooks before mitigation and 0.0097 afterwards.

Rates are clearly highly variable between studies and indeed between years and between trips. This makes it challenging and risky to extrapolate from small sample sizes and may also hinder the accuracy of bycatch predictions.

Melvin et al. (2001) noted extreme inter-annual variation in seabird bycatch by species in Alaska. Seabird bycatch rates were 74% lower in 2000 compared with 1999 in the sablefish longline fishery and 93% lower in the cod longline fishery, driven by changes in seabird behaviour and distribution.

Various authors have explored the fishery related factors that may be associated with this extreme variability in bycatch rates. Melvin et al. (2019) explored several statistical modelling approaches, and found both year and area were important factors, as well as depth, season, total number of hooks and target fish catch per unit effort. The use of streamers and weighted lines in the Alaskan fisheries was associated with a significant reduction in bycatch or all species. Fulmars, however, were recorded bycaught at significant higher rates (by 40%) at night. Dietrich et al. (2009) applied a variety of multivariate models to the extensive observer data set from Alaska and found that 'vessel’ was the single highest contributor to model deviance in 11 or 13 models, explaining 21-25% of deviance. Other fishery related variables rarely contributed more than 10% to explained deviance.

Fangel et al. (2017) used a generalized linear mixed model (GLMM) framework to examine potential drivers in the bycatch of fulmars in the Norwegian inshore Greenland halibut longline fishery. Contrary to the work of Melvin et al. (2019), they found no convincing effect of the use of bird scaring lines on bycatch rates but did note a very significant effect of hook type on bycatch rates. They noted that much of the variation in their bycatch data appears to be random and remains unexplained by any of the measured variables. They conclude that “the bycatch in general is difficult to predict from the spatio-temporal, environmental and other mitigation variables included in our analysis, which suggests that incidental bycatch of fulmars in the Greenland halibut fishery is more random than systematic.”

Against this backdrop of conflicting and generally inconsistent analyses of factors potentially driving fulmar bycatch in longline fisheries, a more pragmatic approach may be to examine what mitigation measures have worked for this and other seabird species in longline fisheries.

1.4 Description of mitigation measures tested and/or used in demersal longline fisheries

Potential options for seabird bycatch mitigation in demersal longline fisheries from the main review papers by Bull (2006), Lokkeburg (2008), Parker (2017) and ACAP (2019) and from a specific study by Fangel (2017) which described a method not covered by the reviews are presented in Table 2. A description of each tabulated method is then provided in the subsequent text. For clarity and based partially on the work of Lokkeborg (2008), we categorised the selected measures into vessel-based, gear-based and non-technical mitigation approaches. Within each of these broad categories we grouped measures with similar mitigation characteristics, such as deterrence, vessel modification and so on, to allow easier conceptualisation of the form and practical considerations of each mitigation approach.

Table 2: Seabird bycatch mitigation measures most relevant to demersal longline fisheries.

Category	General Approach	Specific Approach	Specific Element	Parker 2017	Bull 2006	Lokkeburg FAO 2008	Fangel 2017	ACAP 2019
Vessel-Based	Deterrence	Deterrence during line setting	Bird scaring lines	Y	Y	Y		Recommended
			Water cannon			Y
			Lasers					Not Recommended
			Acoustics			Y
			Olfactory
		Deterrence during hauling	Bird exclusion devices	Y	Y	Y
	Vessel modification	Equipment installation	Underwater line setters	Y	Y	Y
			Line shooter	Y	Y	Y		Not recommended
		Structural modification	Moon pool	Y
	Operational	Vessel operations	Night setting		Y	Y		Recommended
		Crew operations	Offal management	Y	Y			Recommended
Category	General Approach	Specific Approach	Specific Element	Parker 2017	Bull 2006	Lokkeburg FAO 2008	Fangel 2017	ACAP 2019
Gear based	Reduced attraction/ opportunity	Sink rates	External line weighting	Y	Y	Y		Recommended
			Integrated weights	Y
			Branch line weighting					Recommended
		Bait	Thawed bait	Y		Y		Not recommended
	Reduced ‘hookability’	Hook	Size and shape modifications			Y		Not recommended
			Swivel hooks				Y
			Hook shields					Recommended
Non-technical	Input and output measures	Input measures	Effort management		Y	Y		Recommended
		Output measures	Bycatch thresholds

1.4.1 Vessel-based Approaches

Vessel-based mitigation measures include changes to the fishing operations, structural modifications or equipment deployed from the vessel to keep seabirds away from the gear as it is set or hauled. To date the most widely used category of vessel-based mitigation measures in longline fisheries has been physical deterrents deployed during line setting.

Bird-scaring lines (BSL), also known as streamer lines or tori lines, were first developed on Japanese longliners working in the Southern Ocean (Brothers 1999) and are single or multiple lines that are connected to a high point on the vessel and are typically deployed over the stern during line setting operations and are retrieved back onto the vessel after the lines are fully deployed. BSLs have a main rope/s that contain multiple streamers which form a protective barrier over the longline that is designed to deter foraging birds from the vicinity of the baited hooks as the gear is set. The streamers are typically of decreasing length further from the vessel to reduce handling problems that can occur if they entangle the longline. The BSL should be sufficiently long that it extends well beyond the point where the longlines enter the water because baited hooks often remain in foraging range of birds until they have sunk several metres below the surface (ACAP 2016). Numerous studies have shown that the use of single or multiple BSLs significantly reduced seabird bycatch rates in various demersal longline fisheries (Melvin et al. 2001; Lokkeburg & Robertson 2002; Lokkeborg 2003; Paterson et al. 2017) while in another study no clear effect on bycatch rates was found when using a BSL (Fangel et al. 2017). Goad & Debski (2017) also report problems associated with BSLs entangling the longline during setting. BSLs can be tensioned by the deployment of a rope or buoy at the outer end (Goad & Debski 2017; Parker 2017) and this may help maintain the BSL vertically or near vertically above the longline in side-winds, extend its effective range and reduce the likelihood of the BSL becoming entangled with the gear.

The use of water cannons to deter birds from longline vessels during line setting operations was investigated by Kiyota et al. (2001). They used a 30-Kilowatt electric centrifugal pump and reported that the range of the cannon was not sufficient to be particularly effective and that changes in wind direction would further limit its efficacy.

Laser systems have been used to deter birds from fish farms, airports, dairy and other agricultural settings and properties since at least the turn of the millenium (Blackwell et al. 2002; Glahn & Dorr 2000). A marinised version aimed at minimising seabird longline interactions was developed by the Dutch Company Savewave and marketed by Mustad in 2014. The device aims a green laser over the water around the longline as it is being set, and this circle of light (and beam in some conditions) can have a deterrent effect on scavenging seabirds. This device also comes with an optional acoustic deterrent package so that simultaneous acoustic and visual deterrents can be broadcast. A trial by Melvin et al. (2016) concluded that seabirds showed little detectable response to the laser during daylight hours but at night fulmars showed a transient and localized response. No more recent trials appear to have been undertaken (ACAP 2019b). Concerns have also been raised that it may damage seabird eyesight (ACAP 2016), but the evidence for this is not conclusive (Melvin et al. 2016).

Acoustic deterrents are used widely in the terrestrial environment (Gilsdorf et al. 2002), particularly agricultural settings. Gas cannons have been tested is some longline fisheries, but the general perception is that seabirds quickly habituate to the noise and there is little evidence for a long term effective acoustic deterrent for seabirds (Parker 2017).

Although listed in several reviews, there is little empirical evidence for olfactory based solutions. Two studies (Pierre & Norden 2006; Norden & Pierre 2007) from New Zealand suggested that some species of seabirds can be deterred by the use of shark liver oil during fishing operations. Parker (2017) refers to a study in Alaska, where the authors noted that after fish oil deployment behind a trawler the shearwaters departed the area completely. Olfactory approaches do not appear to have been tested or trialled elsewhere and ACAP (2019a) states that there is no evidence of effectiveness in pelagic longline fisheries.

All the methods described above are designed to deter birds from the vicinity of the hooks during standard line setting operations. An alternative approach is to deploy the line and hooks below the surface, so the baited hooks are outside typical foraging depths more quickly.

An underwater line setter was developed and marketed by Mustad, and efficacy tests have been reported by Løkkeborg (1998) and Ryan & Watkins (2002). Although the device showed promise, several problems were encountered. The line setting tube was attached to the transom of the vessel which can rise and fall significantly as the vessel pitches in large waves or swells and the seaward end of the device was frequently lifted out of the water making the baited hooks more visible and available to birds near the boat. Other potential problems included the fact that some of the bird species where the trials were conducted can dive up to 10m below the surface and a line setter extending that deep is impracticable and could cause structural damage in heavy weather. Parker (2017) highlights an example of a longliner in New Zealand that experienced stress on the vessel’s transom due to the presence of a setting tube. In a similar vein, another type of underwater line setter, the Kellian line setter, was conceived by a New Zealand fisherman and has undergone several incremental developments over the last decade (Baker et al. 2016). The basic design involves towing a device just off the stern of the vessel at a depth of 4 to 7m, which the line and hooks pass under increasing the line deployment angle - meaning the hooks are more quickly out of foraging range of most surface feeding seabird species. Trials of the latest version (KLS4.4) also showed promise but some issues with gear deployment and damage and loss of baits was reported. According to Baker et al. (2016), further work is required to address these issues. Despite being conceptually appealing and showing some potential as a mitigation approach, underwater line setters have not proven practicable and are therefore not yet used widely in commercial longline operations.

Another vessel-based approach to increasing the speed baited hooks pass through the foraging zone of surface feeding seabirds is related to mainline tension. Longlines are normally set behind a moving vessel meaning the mainline is under tension so hooks do not sink as quickly as they would if they were dropped into the water on an un-tensioned line.

Hydraulic line shooters reduce or remove tension on the mainline by deploying it more quickly than the vessel is moving (Parker 2017). Studies of the efficacy of line shooting devices on seabird bycatch rates are limited and results are also equivocal. Lokkeborg & Robertson (2002) found that lines set with a line shooter had higher seabird bycatch rates than the same lines set with a BSL, and Robertson (2008) found that sink rates of weighted mainlines were the same whether a line shooter was used or not. As with underwater line setting approaches, line shooters seem a reasonable idea but there is little conclusive evidence that they provide a suitable approach for reducing bycatch in demersal longline fisheries.

Most seabird mortality in demersal longline fisheries appears to occur during line setting operations as birds get caught on baited hooks and are dragged below the surface by the weight of the gear. However, some bycatch also occurs during line hauling and may lead to mortality and injury and some attempts have been made to reduce this interaction.

Bird Exclusion Devices (BEDs), such as brickle curtains, are designed for use when longlines are hauled and provide a barrier of ropes and floats around the area where the lines exit the water to reduce access to baited hooks as they resurface. BED systems were first developed in the mid-1990s in the Patagonian toothfish (Dissostichus eleginoides) fishery in the South Atlantic (Reid et al.2010). Initial trials reported a 97% reduction in interactions during hauling (Snell 2008). Other studies have shown signs that birds may habituate to the device (Sullivan 2004) and the potential for the system to interfere with line hauling operations in heavy weather has also been highlighted (Parker 2017) which might limit the systems utility in high latitude offshore fisheries.

Moon pools involve a vertical tunnel built through the vessels hull that open into a small pool inside the vessel and are often found in drilling ships and scientific research vessels. The basic design has been used in auto-longline vessels in Iceland, Norway, Denmark and the USA (Parker 2017) primarily to provide a safer working environment for the crew compared to open deck vessels. In the context of seabird bycatch, the use of a moon pool would shield the hooks from foraging birds during hauling but would not reduce bycatch that occurs during line setting as lines are usually hauled through the moon pool but are typically set over the stern of the vessel in the traditional manner. Some fisheries also shoot lines through the moon pool. There do not appear to be any direct studies on the effects of moon pools on seabird bycatch rates (Parker 2017).

In addition to more technical bycatch reduction approaches as described above, several studies have assessed how vessel operational changes might affect bycatch rates. Most seabird species forage primarily by sight (though olfaction is known to be used by at least some bird taxa) so in general bycatch rates tend to be lower when lines are set in darkness (Weimerskirch et al. 2000), and night setting is recommended as best practise by ACAP and some national authorities including New Zealand. However, night-setting at high latitudes during summer months is almost impossible when there is little darkness. Furthermore, some seabirds are able to forage effectively in bright moonlight, while others may use the light from deck lights to aid foraging. Conclusive evidence that night setting is a useful measure to prevent fulmar bycatch is lacking and Melvin et al. (2019) concluded that for most seabird species bycatch rates in Alaskan demersal longline fisheries were lower at night, but northern fulmars were the exception and were caught at higher rates (+ 40.4%) at night.

It is widely recognised that the main reason seabirds are attracted to fishing boats is the enhanced foraging opportunities provided by the disposal of discards and offal (Weimerskirch et al. 2000). Several national authorities (e.g., New Zealand) have developed standards making offal management a key part of any mitigation strategy by ensuring offal is not discharged at the same time as lines are being set or that offal disposal during hauling operations is carried out on the opposite side of the vessel. Offal retention (for subsequent disposal when not setting or hauling is occurring) is recommended by ACAP (2019), but it has been highlighted that there may be logistical, or safety constraints associated with the temporary storage of all offal onboard (Bull 2006).

1.4.2 Gear-based Approaches

A factor known to influence seabird bycatch rates in longline fisheries is the speed at which the baited hooks sink through the upper part of the water column during line setting operations (Bull 2006; Lokkeburg 2008; Parker 2017; ACAP 2019). ACAP (2014) produced guidance on minimum line sink rates of 0.3 m/s required to reduce opportunities for surface and near surface feeding species to access baits. Increasing line sink rates above 0.3 m/s is suggested as part of a suite of complementary measures which are used concurrently, and it is not recommended as a standalone approach to bycatch mitigation.

Under commercial conditions sink rates are likely to vary due to factors such as prevailing sea state, effects of upwellings caused by a vessels propellor, buoying effects of bycaught birds and variable materials, shapes and sizes of the weights used (ACAP 2014). In general, research into the effects of line sink rates has either focused on recording line sink rates directly, or by assessing changes in bycatch rates under different weighting configurations (Bull 2006).

Robertson (2000, in Bull 2006) assessed sink rates under different external line weighting regimes by placing 6.5kg weights at various spacings (30m, 50m, 70m, 100m, 140m and 200m) in the demersal longline fishery for Patagonian toothfish near the Falkland Islands. As might be expected, overall sink rates decreased as spacing between weights increased. The sink rate in the top part of the water column was highest with weight spacings of 30m and 50m. Sink rates through the water column did not vary much when weights were spaced 70m or above. A study investigating different weighting regimes (4.25kg, 8.50kg and 12.75kg at 40m spacings) in the Patagonian toothfish fishery around South Georgia found a significant reduction in seabird bycatch rates when 8.50kg rates were used compared to 4.25kg weights, but no additional reduction was achieved by using 12.75kg weights (Agnew et al. 2000, Bull 2006).

Melvin et al. (2001) assessed the effects of adding weights to demersal longlines in fisheries targeting Pacific cod (Gadus macrocephalus) in the Bering Sea and sablefish (Anoplopoma fimbria) in the Gulf of Alaska where northern fulmar are the most frequently bycaught seabird species. In the first year of the study the addition of 10lb (4.5kg) weights every 90m in the cod fishery and 0.5lb (0.25kg) weights every 11m in the sablefish fishery reduced overall seabird bycatch rates by 76% and 37% respectively. However, in trials the following year when the same weighting regimes were compared against bycatch reduction rates associated with the use of paired BSLs no significant reduction was seen. Marked differences in seabird abundance, bait attack rates and bycatch rates were seen between the two years and the authors concluded that extreme inter-annual variation in rare event phenomena such as seabird bycatch has important implications for fisheries management, and that adequate evaluation of seabird bycatch deterrents via observer programs will require multi-year data sets.

The use of integrated weighting in mainlines was tested in a demersal longline fishery in New Zealand (Robertson et al. 2004; Robertson et al. 2006). A comparison between lines with integral weighting of 50g/m and standard unweighted lines produced significant reductions of 95%-98% in bycatch rates of white-chinned petrels (Procellaria aequinoctialis) and 60%-100% reductions for sooty shearwaters (Ardenna grisea). Commercial catch rates were not affected.

In trials in the Alaskan demersal longline fishery targeting cod in the Bering Sea, three experimental mitigation treatments (integral line weighting, integral weighting with BSLs and unweighted lines with BSLs) were compared with a control of no mitigation (Dietrich et al. 2008). Integrated weighting reduced bycatch rates of surface feeding species (northern fulmar and Larus spp.) by 91% to 98% and a diving seabird, the short-tailed shearwater (Puffinus tenuirostris) by 80% to 87%. It was also estimated that integral weighted lines reduced the distance behind the vessel that birds had access to baits by almost 50% when compared to non-weighted lines.

Robertson et al. (2004) recorded link sink rates of unweighted lines and weighted lines made from two materials – silver line and polyester. Tests were carried out from two chartered vessels and no differences in sink rates were found between vessels, but statistically significant differences were observed between line types. The weighted polyester line sank fastest (mean 0.272 m/s), followed by the weighted silver line (0.239 m/s) and the unweighted line (0.109 m/s). However similar sink rates to the weighted lines were achieved by attaching external weights to unweighted lines. In contrast to the findings of Roberson et al. (2004), Pierre et al. (2013, in Parker 2017) found that line weighting configurations and corresponding sink rates varied greatly between vessels operating under normal commercial conditions, which suggests that there could be significant inter-vessel variation in bycatch rates in some fisheries.

The pros and cons of line weighting (external and integral) approaches for reducing seabird bycatch were summarised by Bull (2006) and Parker (2017) and those of relevance to demersal fisheries are tabulated in Table 3.

Table 3: Pros and cons of line weighting approaches for seabird bycatch mitigation (Adapted from Bull (2006) and Parker (2017)).

Pros: There is evidence that optimal line weighting configurations do reduce seabird bycatch.

Cons: There are concerns for crew health and safety associated with the use of extra or heavier external weights.

Pros: Reduced bycatch and attack rates associated with optimal weighting configurations will lead to less bait loss and may therefore translate into improved target catch rates.

Cons: Use of lead weights (integral or external) increases the risk of this potentially harmful compound accumulating in the marine environment.

Pros: Integral weighted lines are safe for crew to use.

Cons: Adding extra weights increases crew workload.

Pros: Integral weight lines have a uniform sink profile which eliminates lofting associated with external weighted lines ( ACAP, 2016).

Cons: Integral weight lines are typically used with auto-lining systems so may not be suitable for all vessel configurations.

Pros: Appropriate weighting can maintain hooks at the correct depths so may improve target catch rates.

Cons: The use of appropriate external weighting regimes can only be checked through at-sea inspections.

Pros: Catch rates of target species were not reduced using integral weighted lines.

Cons: Integral weight lines may lead to higher catches of unwanted fish and elasmobranchs because the main line sits on the seabed.

Pros: The use of integral weighted lines can be checked in port inspections.

Cons: Increased gear costs.

Branch line weighting approaches to bycatch mitigation are normally associated with pelagic longline fisheries where snood lengths are much longer than in demersal fisheries, and so additional weighting will sink the bait more rapidly out of the range of surface feeding birds and keep it below the normal foraging range of diving seabirds. To date there seems to be little research done on branch line weighting in demersal fisheries where snood lengths are short, and baits may be quite close to the seabed when actively fishing so could become snagged if additional weights are added. However, a new longline configuration was developed in 2005 in Chile to try and reduce cetacean depredation in a demersal fishery (Moreno et al. 2007), that is conceptually similar to a branch line weighting approach. The system has been termed the “Chilean longline” and consists of a mainline off which hang weighted branch lines of 15m long at approximately 40m spacings. From each of these branch lines are multiple snoods with baited hooks. One of the inadvertent but positive side effects of using this system was that seabird bycatch was eliminated during the three-year trials of the system (Moreno et al. 2007). The observed bycatch reduction was considered to be entirely associated with the very high initial sink rate that occurs because each branch lines has individual weights of 4kg-10kg.

Investigations into the potential bycatch reduction effect of using thawed rather than frozen baits have largely focused on pelagic fisheries but may have relevance to demersal fisheries. Parker (2017) provided a short summary of work that has been conducted in this area. Two studies (Brothers et al. 1999; Klaer and Polacheck 1998) indicated that thawed baits sink faster, one tested actual sink rates (they also found that swim bladder state affected sink rates) and the other compared seabird bycatch rates from thawed and frozen baits and assumed because rates were lower with thawed baits that sink rates must therefore be higher. However, when Robertson et al. (2010) tested thawed versus frozen bait sink rates they found only a negligible effect and concluded that there would be no significant reduction of seabird bycatch rates with thawed baits. Some issues associated with the use of thawed baits highlighted in Parker (2017) are that: baits may not be fully thawed before deployment, there is a lack of evidence of efficacy of this approach across bait types; thawed baits may detach from hooks more easily and bait thawing requires a specific part of the vessel to be set aside for this task.

Li et al. (2012) developed generalised linear models to examine the effects of different hook sizes and shapes from longline data collected under the US National Marine Fisheries Service (NMFS) Pelagic Observer Programme in Atlantic waters. Four combinations of hook shape and size (8/0 J-hook; 9/0 J-hook, 16/0 circle hook; 18/0 circle hook) were used in the sampled hauls. Results indicated that combinations of hook type and size significantly influenced the probability of catching seabirds. The 8/0 J-hook (the smallest of the four types) was associated with the highest bycatch probability. Both sizes of circle hook had the lowest bycatch probability, but the authors state that results may be confounded by other factors such as bait type, location, season and target species and the relatively low number of recorded seabird bycatches in the analysed dataset. Similarly, a study of the variables affecting seabird bycatch (including black-browed albatross (Diomedea melanophris), white-chinned petrel (Procellaria aequinoctialis) and southern giant petrel (Macronectes giganteus)) by Argentinian and Chilean vessels targeting Patagonian toothfish found that hook size was an important source of variation in bycatch rates (Moreno et al. 1996). A significant inverse relationship between hook size and bycatch rate was found.

A study by Fangel et al. (2017) analysed three years of data from a small-vessel demersal longline fishery for Greenland halibut (Reinhardtius hippoglossoides) in coastal water of northern Norway. Most of the seabird bycatch in the fishery is of northern fulmar. Using statistical models to explore the data, they found no significant trends related to environmental, spatial or temporal factors that could explain the variation in bycatch rates. However, they did find that trips where non-swivel hooks were used had bycatch rates about 100 times higher (mean= 0.760, SE =0.160) than trips which used swivel hooks (mean 0.008, SE 0.002). Another interesting finding was that about two-thirds of the bycaught birds were adults, and that males dominated (71.1%). Beck et al. (2020) also reported a strong sex bias in fulmar longline bycatch in Alaska, where 66% of bycaught fulmars were male. The Fangel et al. (2017) study did not determine the reason for the lower rates associated with the use of swivel hooks but the authors suggest it might be related to the bait behaving less predictably in the water on a swivel hook, the swivel hooks might have a lower hooking efficiency for fulmar (or surface feeding seabirds in general), or the swivel hooks may sink more rapidly because they weigh 6g compared to 2g for the non-swivel hook. Fangel et al. (2017) comment that given the apparently significant reductions associated with the use of swivel hooks observed in this fishery, further research should be undertaken in other fisheries to assess if this finding applies elsewhere.

Much of the research into hook related approaches to seabird bycatch mitigation has focussed on the efficacy of what are generically termed hook shields. The basic premise of hook shields is that a weighted plastic or metal case/capsule covers the hook point and barb during line setting but then releases the hook at a predetermined depth below the likely foraging range of surface feeding or diving birds. To date, most of the investigations into the utility of hook shielding devices have focussed on pelagic longline fisheries with generally positive results (Sullivan et al. 2017, Goad et al. 2017 (in Debski et al. 2018), Jusseit 2010). However, given the significant operational differences between pelagic and large scale demersal longline fisheries it is not clear if hook shields have a useful role to play in bycatch mitigation in demersal longline fisheries.

1.4.3 Non-technical approaches

Non-technical approaches to bycatch mitigation are often classified as either regulatory (top-down) or voluntary (bottom-up) and may involve the development and implementation of input measures (also known as process standards) which aim to control or alter the fishing activity in some way or output measures (also known as performance standards) which aim to control the result of that fishing activity (Morrison, 2004; Squires et al. 2021). Non-technical approaches can be used in isolation or in combination with technical approaches such as those described in the previous sections.

Input measures typically involve management regimes that place restrictions on the level of fishing effort (such as restricting gear dimensions or the number of days vessels are permitted to fish) or by managing the spatio-temporal distribution of effort.

Input measures that reduce effort levels could be successful at reducing bycatch if there is a clear and predictable relationship between effort and bycatch levels, but that relationship is not always straightforward for taxa such as seabirds that are mobile, wide-ranging and exhibit relatively rare but often clumped patterns of bycatch (Baerum et al. 2019; Northridge et al. 2020). Simple effort reduction approaches would almost certainly reduce commercial catches so could significantly affect the economic viability of the fishery and adequately describing those economic consequences is beyond the scope of this review.

However, an increasingly widely advocated input measure for conservation purposes are spatio-temporal closures, which have been introduced in some parts of the world specifically to reduce bycatch of various highly mobile taxa, but generally with mixed results.

Croxhall (2008) describes the development of albatross bycatch mitigation measures in the Patagonian toothfish longline fishery in the Southern Ocean from the 1990’s to the early 2000’s. Most of the mitigation measures employed were technical approaches (BSLs, line weighting, offal management) but a seven-month closed season was introduced around South Georgia in 1995, which was subsequently extended to nine months a few years later. No closure was introduced in the Southern Indian Ocean fishery, but the same technical approaches were used. Croxall (2008) reports that equivalent bycatch reduction in the Indian Ocean fishery took longer to achieve than in the South Atlantic fishery, suggesting this is at least partly because of the absence of the closed area in the Indian Ocean example.

Spatio-temporal closures have also been considered for a range of other mobile and widely distributed species. Murray et al. (2001) assessed the effectiveness of a large scale but short-term (1 month) closure off New England (U.S.A.) to reduce harbour porpoise (Phocoena phocoena) bycatch using at-sea observer data to compare bycatch rates before, during and after the closure and concluded that the measure was ineffective because of spatio-temporal variation in patterns of bycatch rates and the effects of displaced fishing effort. Grantham et al. (2008) evaluated the cost/benefit of three different closure approaches for reducing seabird, shark and turtle bycatch in a longline fishery in South Africa and found that temporary, rather than permanent or seasonal closures, were the most effective at reducing bycatch and minimising costs to industry. Using computer simulations Smith et al. (2021) found that only dynamic (as opposed to static) spatio-temporal closures had the potential to reduce bycatch of the highly mobile leatherback turtle (Dermochelys coriacea) in pelagic longline fisheries, and that relatively high levels of observer coverage of 20% was the minimum required to provide an evidence base for implementing effective closures. Pinn (2018) reviewed the evidence for the conservation benefits of Marine Protected Areas (MPAs) for cetaceans. This review concluded that in most cases MPAs failed to achieve conservation goals because of changes in the spatial distribution of populations for which the protected area was designated, a lack of enforcement or management measures within the MPA and the need for additional measures beyond the MPA boundary. O’Keefe et al. (2014) evaluated the effectiveness of a variety of commercial and non-commercial species bycatch mitigation approaches, including spatio-temporal closures, by a meta-analysis of the results of multiple studies. They found that many of the closures considered did not successfully reduce bycatch, and in some cases created new bycatch issues due to the unforeseen effects of fishing effort displacement. Those closures that successfully reduced bycatch were well designed, typically involved industry input, were economically viable, involved a suite of measures and gave due consideration to possible unintended consequences (O’Keefe et al. 2014).

The use of output measures involves the development of some predefined acceptable bycatch level or required bycatch reduction but the actual means of achieving such a standard is not necessarily prescriptive (Squires et al. 2021) but would be implemented in terms of a bycatch quota or allowance. The use of output measures in relation to bycatch has typically involved the development of a bycatch threshold (or limit or target). Thresholds can either be heuristic, such as the ASCOBANS 1.7% of population abundance human induced mortality rate for harbour porpoise (Scheidat et al. 2013) or explicitly designed based on the biological characteristics of a specific population, such as Potential Biological Removal (PBR) which is used under the USA Marine Mammal Protection Act (Punt et al. 2020) and has been proposed for use for seabirds by OSPAR and HELCOM. Bycatch thresholds can be highly sensitive to the input values used in the calculations and so important pre-conditions for the effective use of thresholds are 1. the existence of clear and agreed management or conservation objectives and 2. robustly defined assessment units. In practical terms implementing output measure-based approaches would require the distribution of a bycatch “quota” across vessels and fisheries to ensure that bycatch levels were within the accepted thresholds.

1.5 Discussion

Previous studies have not revealed any clear and consistent risk factors associated with fulmar bycatch among longline fisheries. Studies in Alaska found that fulmar behaviour and distribution can change sharply year to year, and statistical models suggested that among various fishery related factors, vessel identity did most to explain observed variation in bycatch. Setting at night also increased fulmar bycatch in Alaska substantially. In Norway, the use of swivel hooks was associated with much lower fulmar bycatch rates, but no other fishery related factor stood out as being important. Nevertheless, the numerous technical approaches to reducing seabird bycatch in longline fisheries that have been trialled around the world and summarised above provide a basis for identifying plausible approaches to addressing this issue in relation to fulmars in UK waters.

The technical mitigation approaches reviewed above have met with varying degrees of success, highlighting the point made by Melvin et al. (2019) and many others that “conservation measures should be fishery specific”. The issue is complicated by the fact that birds are highly mobile and subject to behavioural changes that affect how likely they are to get caught.

The majority of seabird bycatch in longline fisheries involves capture on a baited hook which occurs during line setting or less frequently during hauling operations if the bait is still in place. There are occasional reports of birds becoming tangled in snood lines (Moreno 1996), but this appears to constitute a much lower risk. This has led to an emphasis on the development of longline mitigation approaches that either deter birds from the vicinity of the hooks as lines are set, or through methods to ensure that lines are quickly out of the foraging depth range of the species in question.

Bird-scaring lines (BSLs) have been tested successfully in numerous fisheries and are currently legally required in some longline fisheries, including in the USA, Brazil and Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) waters. The specifications for the BSLs in these fisheries are described within the relevant regulations and are fishery specific but have some common attributes that aim to maximise bycatch reduction rates through requirements for line attachment heights, line length and streamer length etc. The recommended configurations are also designed to minimise operational problems that can occur, such as BSLs tangling with the longlines during line setting and so reduce time/cost implications for industry which can help encourage uptake and/or compliance. By reducing bycatch levels and bait loss, the use of BSLs may also increase fish catch rates which would be attractive to industry.

Discussions with industry, and observer data, indicate that BSLs are already being used on a voluntary basis by at least some of the vessels in the UK offshore longline fleet, but that problems with entangling the longlines are quite regularly experienced, and the BSLs do not appear to be very effective in some weather conditions. A logical next step might be to test the designs of BSLs that are already used successfully in other similar fisheries to evaluate if they are appropriate for use in the UK fishery.

Other deterrent approaches such as water cannons, lasers, acoustic deterrents and olfactory deterrents appear to be less promising and there is little evidence to support the testing or use of these mitigation approaches in the UK offshore longline fishery.

Several approaches have been developed to try and ensure that baited hooks are deployed below the foraging range of surface feeding birds. Various versions of underwater line setters have been tested and although the underlying principle is logical, most attempts have encountered a variety of problems including the lower end of the device lifting out of the water in heavy weather (which is prevalent in Scottish waters) making the baits available to birds (Løkkeborg 1998; Ryan & Watkins 2002), to more serious issues of structural damage to vessels (Parker 2017). The consensus appears to be that although conceptually appealing, the development of underwater line setters requires significant future work to make sure they are effective and safe in all sea conditions. At present they do not provide a realistic option for mitigation in the UK fishery.

Line shooters that reduce tension on the mainline by deploying it more quickly than the vessel is moving allowing the hooks to sink more rapidly is an alternative approach, but there is little evidence that the approach significantly reduces bycatch rates or is suitable for large scale demersal longline fisheries. Studies into the use of thawed rather than frozen baits as a means of increasing hook sink rates have produced contrasting results (Brothers et al. 1999; Klaer and Polacheck 1998; Robertson et al. 2010) and there is little evidence that this is a suitable approach for reducing bycatch in a large-scale demersal fishery.

Altering the longline weighting configuration to increase line sink rates, either through changes to the external weights or the use of integral weighted lines, has been shown to reduce seabird bycatch in some fisheries (Agnew et al. 2000; Robertson et al., 2004; Bull 2006; Robertson et al. 2006; Dietrich et al. 2008). However, Melvin et al. (2001) had mixed results in line weighting trials spanning two years and concluded that high inter-annual variation in seabird abundance, bait attack rates and bycatch rates meant that proper evaluations of the effects of line sink rates should be conducted over multi-year periods. ACAP (2014) currently recommend a line sink rate of over 0.3m/s to reduce foraging opportunities for surface/near surface feeders but do not recommend line sink rates as a standalone approach to bycatch mitigation, because under commercial conditions sink rates can vary for a variety of reasons. A short study to quantify line sink rates (Rouxel 2020) was carried out on a single vessel from the UK offshore longline fleet and found that sink rates varied widely depending on the section of the gear, with sections nearer floats sinking more slowly than sections nearer the weights as would be expected. Most of the recorded sink rates were below ACAP guidance, but the study was limited in its scope and these finding may not apply across the fleet and in different conditions. The study does highlight some potential for increasing line sink rates to reduce bycatch in the fishery, but this would require fleet wide assessment of current sink rates and subsequent simulation and significant field testing of modified weighting regimes or complete gear redesigns, and this approach does not appear to be favoured by industry at this time (pers. comm. M. Hermida).

Surprisingly, given that the hook is so prominent in the occurrence of bycatch, there have been relatively few studies investigating how hook designs may influence bycatch rates, and ACAP (2016) did not recommend hook design approaches to mitigation simply because they are insufficiently researched. Perhaps one of the reasons for the lack of studies to date is that the hooks are also of fundamental importance to target species catch rates, so it is possible that there is less inclination to test modifications to that element of the gear. However, there are two interesting findings in relation to the effects of hook size and type in demersal longline fisheries. In the fishery for Patagonian toothfish a significant inverse relationship was found between hook size and seabird bycatch rates (Moreno et al. 1996) and in a Norwegian fishery for Greenland halibut a 95% reduction in fulmar bycatch rates was reported with the use of swivel circle hooks compared to the J hook design that was used as standard in the fishery (Fangel et al. 2017). The exact reason behind the large reduction associated with the use of swivel hooks was not fully determined in the study, and further funding was not secured to extend the work (pers. comm. K. Baerum), however this seems an approach potentially worthy of closer examination in the UK fishery.

Other hook related approaches involve changing the hook colour, altering the hook position in the bait and shielding the hook point during line setting, but as with bait related approaches, most of the research into the effects of these hook modifications have been undertaken in pelagic longline fisheries and the findings do not appear relevant to large scale demersal longline fisheries.

Adapting the operational behaviour of fishing vessels is another approach that has been investigated in relation to seabird bycatch. Night setting is recommended as best practise by ACAP and the New Zealand fisheries management authority. However, work by Melvin et al. (2019) found that for most seabird species bycatch rates associated with night setting were lower, but this did not apply to northern fulmar which showed significantly higher bycatch rates (+40%) in night set operations. This certainly calls into question the relevance of the ACAP recommendation in relation to the UK fishery, where fulmar appear to constitute over 95% of the total seabird bycatch. If the findings of Melvin et al. (2019) are transferable to the UK situation, then a requirement for night setting would likely increase overall bycatch in the fishery. Night setting approaches are also hampered by the shorter night-time period from spring to autumn in high latitude fisheries, such as the longline fishery off Scotland.

A second commonly proposed operational approach is offal and discard management. Seabirds are attracted to fishing boats because they provide a reliable source of food. Daily operational patterns in the UK longline fishery tend to involve a long period of hauling lines and processing the catch, during which offal and discards are disposed of, and then a shorter period of line setting during which most fatal bycatch occurs. This means there is not a clear temporal overlap between offal and discard disposal and the period of highest bycatch mortality risk. However, some bycatch does occur during line retrieval, and this might be reduced by altering where or when the offal/discards are disposed. For example, this might be achieved by routing disposal chutes to exit through the opposite side of the vessel from where hauling occurs, or by keeping offal/discards aboard and disposing of them when lines are no being set or retrieved. Disposing of offal away from the hauling area seems logical and would be possible to implement without major disruption if the deck layout of the vessel was suitable. There may be logistical, or safety issues associated with keeping offal/discards on board for later disposal, as highlighted by Bull (2006), and these would need to be considered carefully before such an offal/discard management approach was implemented in the fishery. This approach might help break the behavioural link between fishing operations and food resource that some birds have clearly learned. However, it is worth considering if the approach of retaining offal/discards on board could in fact increase bycatch rates, if birds began to forage at higher density and more aggressively around the line hauling/setting operations because the previous steady supply of offal/discards had been removed. Sherley et al. (2019) estimated that reductions in fisheries discards in the North Sea between 1990 and 2010 may have led to a 39% reduction in the number of scavenging seabirds that this resource could support. Clearly, some thought should also be given to how changes in offal/discard management might affect the foraging success of those birds that have learnt to feed around longline fishing vessels and what the population impacts of reducing that foraging opportunity might be.

Some bycatch occurs in the UK fishery during line retrieval and in most cases the birds are released alive but will have some level of injury and post release mortality rates are not known. Some measures such as bird-exclusion devices are designed to keep birds away from the line hauling area but the results in terms of bycatch reduction are somewhat conflicting, with some studies reporting reduced interactions (Snell 2008) and others indicating that birds habituate to the presence of the device (Sullivan 2004). Some operational problems where the device tangles with the lines as they are hauled have also been reported (Parker 2017), particularly during use in heavy weather and this would likely constitute a significant safety issue of the vessel and crew. Given that most fatal seabird interactions with the UK fishery are associated with line setting operations the utility of bird-exclusion devices to significantly reduce bycatch mortality is probably limited.

The use of input measures to reduce seabird bycatch would require a significant management effort to firstly define clear species-specific management/conservation objectives, then detailed quantitative assessments using robust data to evaluate how those objectives might best be achieved and this should include a proper consideration of unintended consequences, and an economic impact assessment to understand the financial impacts on industry and possible changes in food supply to society. Output measure approaches would also require the defining of clear management objectives and the setting of bycatch thresholds, but perhaps the most difficult issue is to determine and agree how action to reduce levels to below agreed thresholds (assuming bycatch levels exceed the threshold) would be distributed across the vessels, fisheries and nations that contribute to the known mortality of what are typically highly mobile species with trans-national distributions.